The present invention relates to a cooler for cooling material that has been treated in a rotary drum kiln, such as burnt lime, which cooler comprises at least two cylindrical housings positioned one inside the other, surrounding the kiln and rotating with it, which housings have been mounted at the discharge end of the kiln concentrically therewith and between which housings there is formed an annular space, the innermost cylindrical housing being fastened to the kiln at the ends of the housing, and the annular space having a feed inlet in connection with the discharge outlet of the kiln for transferring hot material from the kiln into the cooler and a discharge outlet for discharging cooled material, in which annular space the material flows counter-currently in relation to the cooling gas flow.
The material that has been treated in a rotary drum kiln has typically been cooled in so-called satellite coolers. In this cooling system, a number of cooling cylinders are attached stationary at their inlet ends by means of a drop duct to the kiln shell around the circumference of the kiln. They operate in counter-current principle heating the combustion air going to the kiln. Nowadays the capacity of rotary kilns, such as lime sludge reburning kilns, has risen so high that satellite coolers are no more capable of treating the whole production satisfactorily. The velocity of the air in the drop ducts, the re-entering of material to be cooled back into the kiln, the mass of the coolers, lime dust emission etc. problems are very difficult to control.
It has been possible to essentially decrease the number of this kind of problems by using a so-called sector cooler used in the lime sludge reburning kiln. A sector cooler is a device located outside the shell of a drum kiln and formed of two cylinders disposed one inside the other, which device rotates together with the kiln. It is supported to the kiln shell stationary at its inlet end and by way of motion joint at its discharge end. The object of the cooler is the cooling of hot material passing from the kiln, such as burnt lime generated from lime sludge and the pre-heating of secondary air entering the kiln, respectively. The outer and inner housings of the cooler are joint together by means of elongated plates arranged in the radial direction, whereby the annular space formed between the housings may be divided into the desired number of cooling sectors. At its inlet end the cooler is welded stationary to the kiln shell via drop ducts. Through the drop ducts, hot material is led from the kiln to the inlet end of the cooler. The inlet end is provided with a conical part, receiving the material passing via the drop conduits. The cooler is surrounded by a radiation shield stationary connected to a discharge hopper for cooled material, the objective of which shield is to guide the cooling air in the desired way into the cooler, to prevent dust emission and also to improve the efficiency of the cooler.
The material of the section cooler may be usual low-alloy heatproof ferrite steel, due to which the differences in thermal expansion between the cooler and the kiln shell remain reasonable. Typically, this kind of construction steel is heat-resistant even up to temperatures above 500xc2x0 C. In a burnt lime cooler, typically more than half of the cooler""s length operates in a temperature range where the temperature remains below 550xc2x0 C.
The cross-section area of the drop ducts may be easily increased, compared to e.g. those of a satellite cooler, by increasing the width of the conduit towards the longitudinal direction of the kiln. This way the velocities of the secondary air may be maintained within admissible limits, whereby even fine particles are easily guided into the cooler. The number of the drop ducts, as well as the sectors, is dependent on the diameter of the kiln. The more sections, the better cooling efficiency is obtained.
The supporting of the section cooler at the discharge end for material to be cooled, e.g. burnt lime, is arranged by means of supporting members fastened to the inner housing of the cooler, which members support the cooler to the kiln shell, allowing for movements caused by temperature differences between the kiln shell and the cooler.
Due to the very high temperature of the material passing from the kiln to the conical part of the cooler, e.g. the temperature of burnt lime is typically between 950 . . . 1050xc2x0 C., the conical part of the cooler and the first part of the section zone is subjected to a strong thermal stress, which the ferrite steels used in these constructions are not always capable of resisting well enough. Therefore, it is preferable to protect the conical part of the cooler and the first part of the section zone by means of a fireproof inner lining. The resistance of the in-wall may be decreased, though, due to differences between the properties of materials used in the in-wall, on the one hand, and in other parts of the cooler, one the other hand, e.g. thermal expansion coefficients.
The cooler may also be constructed completely of such a material that resists the operating temperatures. This kind of austenitic cooler is well resistant to thermal stress caused by hot material, such as burnt lime. However, attention must be paid to the control of thermal movements between the kiln casing and the cooler made of various materials. In such a case, the cooler must be provided with supporting elements allowing for radial movements, which results in complicated and expensive supporting constructions. Furthermore, a cooler made completely of austenitic material is expensive.
The object of the present invention is to provide a cooler construction mechanically superior to prior art solutions, which construction minimizes the stresses in construction materials caused by both mechanical loads and thermal movements. In addition to that, the cooler is to be economical.
In order to meet these requirements, a characteristic feature of the cooler according to the present invention as stated in the preamble is that the annular space comprises at least two sections so that each cylindrical housing comprises at least two cylinders located sequentially in the direction of the longitudinal axis of the kiln and attached to each other and that the innermost cylindrical housing comprises means for fastening the cooler to the kiln essentially at the junction point of the sequential cylinders, too.
An essential characterizing mechanical feature of the new cooler is that the stationary supporting and fastening points between the cooler and the kiln shell (the supporting and/or fastening point is understood here and after as a plane perpendicular to the longitudinal axis of the kiln, on which plane the individual supporting or fastening points are essentially located both on the kiln shell and in the cooler) are located in the direction of the longitudinal axis of the kiln essentially at the central part of the cooler. In this way, the supporting forces may be distributed to three points instead of the previous two.
Additionally, this makes it possible to divide the cooler in the longitudinal direction into at least two separate parts. Preferably, the annular space forming the cooler has two sections, whereby, in the flow direction of the material to be cooled, the first cylinders disposed one inside the other form the pre-cooler, which receives the hot material passing from the kiln, and the following cylinders disposed one inside the other form the after-cooler, wherefrom the cooled material is discharged. The joining point of the sequential cylinders is located, in the direction of the longitudinal axis, essentially at the central part of the annular space. In connection with this invention, the central part of the cooler, or the annular space, means a zone, which is wider than the mid-point of the length in the direction of the longitudinal axis and its immediate vicinity. The central part covers a zone extending at both sides of the mid-point typically to a length of 40-60% of the total length of the cooler and on which the joining point of the pre- and after-coolers is located. Usually the pre-cooler is shorter than the after-cooler, but depending on process conditions the lengths may be different as well. In a lime sludge reburning kiln, the length of the pre-cooler is typically 30-40% of the total length of the cooler. An essential characteristic of the invention is that by means of a cooler construction according to the invention an additional supporting point is provided in addition to the fastening points at the ends.
Thus, the pre-cooler is a device with cylindrical outer and inner housings and made of suitable heat-resistant material, preferably austenitic material, and located in the inlet part of the cooler. It is supported directly to the ducts for the material passing from the kiln, which are connected to the kiln shell in radial direction, and at the joining point of the pre- and after-coolers to the after-cooler. The inner cylinder of the pre-cooler is fastened to the ducts so that the supporting points act as sliding surfaces, which allows for radial movements caused by differences in the thermal expansion of various materials and temperature differences. The outlet end of the pre-cooler is supported by means of a light slide joint to the inlet end of the after-cooler, which also allows for the necessary radial and axial movements.
The after-cooler is preferably made of normal non-alloy or low-alloy ferrite steel having a thermal expansion coefficient approximately the same as that of the kiln shell. This makes it possible to weld the inlet end of the after-cooler by means of suitable components directly to the kiln shell, whereon an axial fastening point of the central part of the cooler, a so-called mid-support is formed. The pillaring of the discharge end of the after-cooler is effected directly to the surface of the kiln shell by means of slide supports or corresponding members, which allow for radial motion and are not subjected to remarkable bending stresses. As the kiln casing and the after-cooler are preferably made of the same kind of material, differences in their thermal expansion are so small that no special arrangements are needed in view of the radial motion of the after-cooler.
According to one embodiment of the invention, the inlet end of the after-cooler is supported by means of a sliding joint to the outlet end of the pre-cooler. In that case, too, the so-called mid-support of the cooler is attached to the colder part of the cooler (to the after-cooler). The mid-support is always arranged in that part of the cooler, the material of the inner cylinder of which has essentially the same thermal expansion properties as the kiln shell.
The outer and inner housings are fastened to each other by means of radial longitudinal partition walls.
The outer and inner housings of the pre-cooler section of the annular space are cylindrical along the whole of their length, whereby at the inlet end of the pre-cooler there is no feeding cone, but a xe2x80x9cfeeding cylinderxe2x80x9d. Due to this, the duct ends, through which the hot material enters the cooler, may extend so deep into the cooler that at the same time the passing of the material through the ducts back into the kiln is prevented.